In public wireless systems such as fire defense wireless systems, disaster defense wireless systems, and police wireless systems, a Single Channel per Carrier (SCPC) system that allocates one signal channel to one carrier frequency is used in some cases (the following patent documents).
In such public wireless systems, there are a plurality of mobile stations in a region (communication area) covered by one base station. Communication from a base station to a mobile station is referred to as downlink communication, and inversely, communication from the mobile station to the base station is referred to as uplink communication. Signals used respectively are referred to as a downlink signal and an uplink signal.
For the public wireless systems, there are the following characteristics and requests.
(1) An administrative unit such as a local prefectural government, and a municipal government, or a cooperative or an association in which a plurality of these units are combined is operated as one unit.
(2) For each unit of these, there are main operations in which command information and annunciation information is transmitted from the center to terminals of an administrative organization via downlink communication and a site situation is reported to the center from a terminal site via uplink communication. In other words, in downlink communication, one-to-many simultaneous distribution such as broadcasting is mainly performed, and in uplink communication, one-to-one individual communication is mainly performed.
(3) Therefore, when only downlink communication needs to be performed, a reception-dedicated terminal (command receiver) is used. As such a case, a case is cited in which, for example, a fire company organization is a user in a fire defense wireless system.
(4) On the other hand, in uplink communication, it is desirable that not only a base station but also another mobile station can intercept uplink communication from a specific mobile station to share information between the mobile stations.
To realize a public wireless system having these characteristics and requests, allocation of frequency resources is necessary. There is an allocation method based on a hierarchical structure in which, for example, a frequency band is broadly allocated to each administrative function such as fire defense, disaster defense, and police, and further, in the frequency band, a minute frequency band (frequency channel) for each regional organization is allocated.
In this case, to prevent radio wave interference among wireless communication systems of neighboring regions, it is necessary to allocate different frequency channels to respective regional organizations where radio waves are reachable. Therefore, it is desirable to ensure as many frequency channels as possible. However, the frequency resource is limited, and therefore in the SCPC system, an occupied frequency bandwidth of each frequency channel is suppressed in a range where voice signal transmission is possible, and a digital communication system (digital modulation and encoding) has been introduced as a suppression means.
As a specific example, in a digital fire defense wireless system in Japan, a frequency bandwidth of 6.25 kHz is allocated to one frequency channel, and π/4 shift Quadrature Phase Shift Keying (QPSK) modulation as a digital modulation method, bandwidth compression, and further encoding for information concealment are performed.
Different frequencies are used for downlink communication and uplink communication that configures duplex communication with the downlink communication to become a pair, respectively, but frequency channels of the same band width are basically allocated and the same digital modulation method and encoding method are used.